Molded reflectors for light-emitting diode assemblies

ABSTRACT

Polymer compositions are described containing a poly(1,4-cyclohexanedimethanol terephthalate) polymer in combination with a white pigment and optionally one or more reinforcing fillers. In accordance with the present disclosure, the composition also contains one or more reactive viscosity stabilizers. In order to prevent against yellowing, the composition is free of various aromatic epoxy resins, such as novolac epoxy resins. The polymer composition has excellent reflectance properties making the composition well suited for producing reflectors for light sources, such as LED assemblies.

BACKGROUND

Light-emitting diodes, commonly called LEDs, continue to increase inpopularity as a light source for use in many and diverse applications.The demand for LEDs has grown rapidly, especially in the last fiveyears. LEDs are being used as light sources in numerous applications dueto their many advantages over conventional light sources. LEDs generallyconsume significantly less power than incandescent and other lightsources, require a low voltage to operate, are resistant to mechanicalshock, require low maintenance, and generate minimal heat whenoperating. As a result, LEDs are displacing incandescent and other lightsources in many uses and have found applications, for instance, astraffic signals, large area displays, interior and exterior lighting,cellular telephone displays, digital clock displays, displays forconsumer appliances, flashlights, and the like.

LEDs generally include a light-emitting diode mounted on a substratethat is electrically connected to a lead frame. The lead frame typicallyincludes two terminals for connecting the LED to a power source. Thelight-emitting diode is a semiconductor device fabricated similar to themanner in which integrated circuits are produced. For instance, thelight-emitting diode can be made from several layers of material thatare sequentially deposited on a semiconductor substrate. Thelight-emitting diode within the semiconductor material includes ann-type material separated from a p-type material by an active layer.When a voltage is applied to the diode, positive charges or “holes” fromthe p-type material move towards the active layer while the negativecharges or electrons from the n-type material also move towards theactive layer in an opposite direction which produces light. Inparticular, the moving electrons release energy in the form of photons.Thus, one significant advantage of LEDs is that the devices producelight without a filament that will burn out over time. Thus, LEDs last arelatively long time, can be made to be very compact, and are verydurable. Further, since a filament is not heated in order to producelight, LEDs are also very energy efficient.

After a light-emitting diode is fabricated, the semiconductor chip canbe mounted adjacent to a reflector and connected to a lead frame. Thelead frame can include an anode terminal and a cathode terminal forapplying power to the assembly. In certain embodiments, the LED elementlocated within the reflector can be sealed by a translucent ortransparent resin. The transparent or translucent resin may serve as alens for further enhancing the light that is emitted.

The reflector for the LED can also serve as the housing for the LED andis typically made from a molded polymeric resin. For example, thepolymeric resin can be injection molded to form the housing andreflector. In one embodiment, the polymeric resin is injection moldedover a lead frame for integrating the lead frame into the LED assembly.

The molded polymer resin used to form the reflector preferably possessesa particular combination of characteristics and properties. Forinstance, the polymer resin should be well suited to reflecting light atthe wavelength at which the LED operates. Many LEDs, for instance, aredesigned to emit a white light. Thus, the polymer resin used to form thereflector should reflect a significant amount of light in the visiblelight region and particularly should reflect a significant percentage oflight in the blue light wavelength range. Reflecting light in the bluewavelength range, for instance, has been found to significantly enhancethe brightness of the LED, since LEDs that emit a white light emit asignificant amount of light in the blue wavelength range. Increasing thereflectance of the reflector as high as possible minimizes loss of lightwhen the LED is being operated.

The polymer resin used to form the reflector should also possess a highwhiteness index. The whiteness index of the reflector indicates how wellthe reflector can reflect light over the entire visible light wavelengthrange (from about 400 nm to about 700 nm). In general, the higher thewhiteness index of the material, the higher the reflectance of thematerial. A material possessing a white index value of 100, forinstance, is considered a substantially perfect reflecting diffuser.

In addition to having excellent reflectance properties, the polymerresin used to form the reflector should also have good melt flowproperties during injection molding of the parts. For instance, many LEDstructures are relatively small having dimensions that at times can beless than 1 millimeter. Reflectors can also have relatively complexshapes depending upon the particular application and the geometries ofthe lead plate in the LED. Thus, when the polymer resin is heated, thepolymer should have sufficient flow properties in order to uniformly andrepeatedly fill the interstices of the mold. The polymer resin shouldalso have a stable viscosity that does not fluctuate during processing.

In addition to the above, the polymer resin used to form the reflectorshould have sufficient heat resistance including long term agingstability when either being soldered onto an adjacent part or whenexposed to the operating temperatures of the LED. Many LED assemblies,for instance, are attached to circuit boards and other substrates usingreflow oven welding processes that operate at temperatures up to about260° C. The polymer resin should have good heat resistance properties tothe reflow process and should not blister or otherwise deteriorate whensubjected to the welding conditions.

During use, the LED also generates heat which is absorbed by thereflector. In the past few years, the amount of heat generated by theLED has increased as the LED element power has increased. When subjectedto heat during welding and/or heat during use, the reflectanceproperties of the polymer resin should not deteriorate. In the past, forinstance, exposure to high temperatures and/or repeated heating andcooling during use have caused polymer resins to yellow. Yellowingcauses the whiteness index of the resin to lower. Yellowing isespecially a problem for LEDs that emit blue light since yellow surfaceshave a tendency to absorb light in the blue wavelength range.

In addition to the above, the reflector is generally a thin small partand requires satisfactory mechanical strength. Thus, reflectors shouldalso have sufficient impact strength to avoid breakage during assemblyof the LED and during use of the LED.

In U.S. Patent Publication No. 2007/0213458 entitled “Light-EmittingDiode Assembly Housing Comprising Poly(cyclohexanedimethanolterephthalate) Compositions”, a reflector for an LED is disclosed thatis made from a poly(cyclohexanedimethanol terephthalate) (hereinafter“PCT”) composition. The '458 application, which is incorporated hereinby reference, has made great advances in design and function of LEDs.The present disclosure, however, is directed to further improvements. Inparticular, the present disclosure is generally directed to a reflectorfor an LED made from a PCT polymer composition that has improvedviscosity characteristics and/or reflectance characteristics, includingbetter resistance to yellowing after thermal aging.

SUMMARY

In general, the present disclosure is directed to a molded reflector fora light source, such as a light-emitting diode. The present disclosureis also directed to a polymer composition for producing the reflector.

As will be described in greater detail below, the polymer composition ofthe present disclosure is formulated so as to have a combination of goodmelt flow properties during injection molding and high reflectanceproperties. In addition, the reflectance properties of the polymercomposition are resistant to thermal aging and yellowing over time.

In one embodiment, for instance, the present disclosure is directed to amolded reflector surrounding a light-emitting source. The reflector ismolded from a polymeric material. The polymeric material is comprised ofpoly(1,4-cyclohexanedimethanol terephthalate), a white pigment, and atleast one reactive viscosity stabilizer. In accordance with the presentdisclosure, a reactive viscosity stabilizer is selected that not onlystabilizes the viscosity of the polymeric material during injectionmolding conditions but also does so without later causing yellowing. Inthis regard, the reactive viscosity stabilizer comprises a phenoxy resinor a non-aromatic epoxy resin. To prevent against yellowing, thepolymeric material is also free of any epoxy novolac resins.

In accordance with the present disclosure, the polymeric material usedto form the molded reflector can have an initial reflectance at 460 nmof greater than about 90%, such as greater than about 93%, such asgreater than about 95%. The initial reflectance at 460 nm is generallyless than 100%. The polymeric material can also have an initialwhiteness index of greater than about 86, such as greater than about 92,such as greater than about 95. The initial whiteness index is generallyless than 103, such as less than about 100. Of particular advantage, thepolymeric material can have a whiteness index after aging at 200° C. forfour hours of still greater than about 50, such as greater than about60, such as greater than about 62, such as greater than about 65, suchas greater than about 68, such as even greater than about 70. Ingeneral, the whiteness index after aging at 200° C. is less than theinitial whiteness index of the material and is generally less than about95, such as less than about 85.

In addition to a PCT resin, a white pigment and at least one reactiveviscosity stabilizer, the polymeric material can also contain variousother ingredients and components in various amounts. In one embodiment,for instance, the polymeric material can further contain an inorganicfiller. In one particular embodiment, the polymeric material containsfrom about 20% to about 60% by weight of the PCT polymer, from about 1%to about 40% by weight of the inorganic filler, from about 15% to about50% by weight of the white pigment and from about 0.2% to about 8% byweight of the one or more reactive viscosity stabilizers.

In addition to the above, the polymeric material, in certainembodiments, can further contain one or more other thermoplasticpolymers. Other thermoplastic polymers that may be present in thepolymeric material include a polybutylene terephthalate, a liquidcrystal polymer, or mixtures thereof. The one or more thermoplasticpolymers can be present in the polymeric material in an amount fromabout 1% to about 15% by weight.

In one embodiment, the polymeric material can further contain apolytetrafluoroethylene polymer. The polytetrafluoroethylene polymer canbe present in the polymeric material in an amount from about 0.5% toabout 10% by weight.

The polymeric material may also include one or more impact modifiers.The impact modifiers may be reactive or non-reactive. For instance, inone embodiment, the polymeric material contains a terpolymer ofethylene, methyl acrylate and glycidyl(meth)acrylate.

In another embodiment, the polymeric material contains anethylene-(methyl)acrylate copolymer. In still another embodiment, thepolymeric material may include a combination of the terpolymer ofethylene, methyl acrylate and glycidyl(meth)acrylate and anethylene-(methyl)acrylate copolymer.

As described above, the polymeric material contains a reactive viscositystabilizer that, in one embodiment, may comprise a phenoxy resin. In analternative embodiment, the reactive viscosity stabilizer may comprisean alicyclic epoxy resin.

The polymeric material used to form the molded reflector not only hasgood melt flow properties but also is resistant to yellowing. In oneembodiment, the polymeric material can exhibit a spiral flow of greaterthan about five inches as will be described below.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including thebest mode thereof to one skilled in the art, is set forth moreparticularly in the remainder of the specification, including referenceto the accompanying figures, in which:

FIG. 1 is a perspective view of one embodiment of an LED assembly madein accordance with the present disclosure;

FIG. 2 is a plan view of the LED assembly illustrated in FIG. 1;

FIG. 3 is a perspective view of another embodiment of an LED assemblymade in accordance with the present disclosure; and

FIG. 4 is a perspective view of one-half of a mold used for measuringspiral flow length.

Repeat use of reference characters in the present specification anddrawings is intended to represent the same or analogous features orelements of the present invention.

DETAILED DESCRIPTION

It is to be understood by one of ordinary skill in the art that thepresent discussion is a description of exemplary embodiments only, andis not intended as limiting the broader aspects of the presentdisclosure.

The present disclosure is generally directed to a light-emitting diodeassembly and to a reflector for a light-emitting diode. The presentdisclosure is also directed to a polymer composition that is well suitedfor use in producing light-emitting diode assemblies.

The polymer composition of the present disclosure generally contains apoly(1,4-cyclohexanedimethanol terephthalate) polymer in combinationwith a white pigment. Optionally the composition can also contain areinforcing agent, such as a filler or reinforcing fibers. In accordancewith the present disclosure, the composition further contains one ormore reactive viscosity stabilizers. The reactive viscosity stabilizersassist in maintaining an optimum viscosity of the composition duringmelt flow applications, such as during injection molding. In particular,the one or more reactive viscosity stabilizers inhibit the viscosity ofthe composition from unfavorably fluctuating during melt processing. Ofparticular advantage, the present inventors discovered that theparticular melt viscosity stabilizers of the present disclosure also donot cause unwanted yellowing of the composition during later use,especially when the composition is used as a reflector for a lightsource. In this regard, the composition may also be formulated so as toexclude additives and stabilizers that may cause yellowing to occur. Inthis regard, in one embodiment, the composition is free from some or allof any aromatic epoxy resins, and is especially free of a novolac epoxyresin.

The polymer composition of the present disclosure can be formulated soas to have excellent melt flow properties in combination with excellentlight reflective properties. With respect to the melt flow properties,for instance, in one embodiment, the polymer composition can have aspiral flow length of at least five inches, such as at least six inches,such as even at least seven inches. In general, the spiral flow lengthis less than about fifteen inches, such as less than about twelveinches. As used herein, the spiral flow length is determined at atemperature of 305° C. and at a mold temperature of 120° C. Spiral flowlength is measured by injecting the polymer composition into a mold asshown in FIG. 4. The mold is 1/32 inches thick and ½ inches wide. Thepolymer composition is injected into the mold using a 32 mm extruder atan injection speed of 4 inches per second and a shot size of 1.8 inches.Spiral flow length generally indicates the flow characteristics of thepolymer composition when being melt processed. Higher spiral flowlengths indicate the ability of the material to uniformly and evenlyflow into a mold, which also indicates the ability of the material tofill any interstices of the mold that may exist. For example, a higherspiral flow length is particularly preferred when molding small partsthat may have complex three-dimensional configurations, such asreflectors and housings for LED assemblies.

In addition to having a relatively high spiral flow length, polymercompositions made according to the present disclosure can also beformulated so as to have a stable viscosity. In particular, one or morereactive viscosity stabilizers can be incorporated into the formulationin an amount sufficient for the melt viscosity of the composition duringprocessing to not fluctuate by more than about 5%, such as no more thanabout 3%.

The polymer composition of the present disclosure also has a relativelyhigh initial reflectance, and excellent reflectance stability. Forinstance, once molded into an article, the polymer material of thepresent disclosure can have an initial reflectance at 460 nm of greaterthan about 90%, such as greater than about 93%, such as greater thanabout 95%. Reflectance is measured according to ASTM Test Method 1331using a spectracolormeter. During testing, a CIE D65 daylight illuminantis used at an angle of 10°.

In addition to initial reflectance, polymer articles made according tothe present disclosure can also have a relatively high initial whitenessindex. Whiteness index can be measured according to WI E313. Articlesmade according to the present disclosure can have an initial whitenessindex of greater than about 80, such as greater than about 90, such asgreater than about 92, such as greater than about 95.

Of particular advantage, articles made according to the presentdisclosure also have great reflectance stability properties. Forinstance, after aging at 200° C. for four hours, the whiteness index ofarticles made according to the present disclosure can be at least about70, such as at least about 72, such as at least about 74, such as evengreater than about 75. The whiteness index after aging is lower than theinitial whiteness index.

In addition to the above properties, the polymer composition of thepresent disclosure also has good reflow resistance properties atrelatively high temperatures, such as at temperatures around 260° C. Thepolymer material has good silicone adhesion properties, which may beimportant in applications where an adhesive is used to either attachcomponents in the LED assembly to the reflector or to attach thereflector to a substrate. Articles made according to the presentdisclosure also have good mechanical properties, such as good impactresistance. The material of the present disclosure also displays lowmoisture absorption.

Referring to FIGS. 1 and 2, one embodiment of an LED assembly 10 thatmay be made in accordance with the present disclosure is shown. In theembodiment illustrated in FIGS. 1 and 2, the LED assembly 10 isconsidered a side view LED. As shown, the LED assembly 10 includes alight-emitting diode 12 that is configured to emit light when a currentis fed through the device. The light-emitting diode 12, for instance,may be comprised of a semiconductor chip including multiple layers ofmaterials. The LED 12 generally includes an n-type material layer and ap-type material layer, which form a p-n junction that can be connectedto a voltage source. In one embodiment, for instance, the p-type layermay comprise doped gallium aluminum arsenide, while the n-type layer maycomprise doped gallium arsenide.

The LED 12 is connected to a first bonding wire 14 and to a secondbonding wire 16. The bonding wires 14 and 16 are connected to a leadframe 18. The lead frame 18 includes a first lead frame portion 20 and asecond lead frame portion 22. The lead frame 18 may include or beconnected to an anode 24 and a cathode 26 which may also be considered afirst terminal 24 and a second terminal 26.

In accordance with the present disclosure, the LED assembly 10 furtherincludes a reflector 28 which can also serve as the housing for the LEDassembly. The reflector 28, in accordance with the present disclosure,is made from a polymer composition having excellent reflectanceproperties.

As shown in FIGS. 1 and 2, the reflector 28 defines a cavity 30 in whichthe LED 12 is located. The walls of the cavity 30 generally surround theLED 12 and, in the embodiment illustrated, have a depth sufficient forthe LED 12 to be recessed within the cavity.

The cavity 30 of the reflector 28 surrounds the LED 12 and serves toreflect light being emitted by the LED in an outward direction. Thecavity 30 may have any suitable shape. For instance, the cavity 30 maybe cylindrical, conical, parabolic, or any other suitable curved form.Alternatively, the walls of the cavity 30 may be parallel, substantiallyparallel, or tapered with respect to the diode 12. In the embodimentillustrated in FIG. 1, for instance, the cavity 30 has a smooth surfaceand is comprised of side walls 32 and 34 and end walls 36 and 38. Theside walls 32 and 34 taper in an outward direction from the LED 12. Theend walls 36 and 38, on the other hand, can be substantially parallel ormay also taper outwardly from the LED source.

If desired, the cavity 30 of the reflector 28 may be filled with a clearmaterial, such as a transparent material or a translucent material. Forinstance, the cavity 30 may be filled with an epoxy or a siliconematerial. In one embodiment, the material used to fill the cavity 30 mayact as a lens for the light being emitted by the LED 12.

Referring to FIG. 3, another embodiment of an LED assembly 50 that maybe made in accordance with the present disclosure is shown. In theembodiment illustrated in FIG. 3, a top view LED assembly is shown. Thetop view LED assembly 50 is similar in construction to the side view LEDassembly 10 illustrated in FIGS. 1 and 2.

For instance, the top view LED assembly 50 includes an LED 52 that ispositioned towards the bottom of a cavity 54 of a reflector 56. The LED52 is also connected to a lead frame 58. In the embodiment illustratedin FIG. 3, the cavity 54 of the reflector 56 is filled with a clearmaterial 60.

LED assemblies as shown in FIGS. 1-3 generally have relatively smalldimensions. For example, the LED assemblies typically have a greatestdimension (such as height, width, depth or diameter) that is generallyless than about 10 mm, such as typically less than about 8 mm. The LEDassemblies typically include at least one dimension, such as depth, thatis less than 5 mm, such as less than 2 mm, such as even less than 1 mm.As will be described below, the polymer composition of the presentdisclosure is capable of forming reflectors for LED assemblies usingmelt flow processing techniques. For instance, in one embodiment, thepolymer composition of the present disclosure is blow molded in formingthe reflectors. Of particular advantage, the composition of the presentdisclosure is formulated so as to have melt flow properties capable offorming hundreds of reflectors simultaneously.

As described above, the polymer composition of the present disclosurecontains a poly(1,4-cyclohexanedimethanol terephthalate) polymer, whichis typically referred to as a “PCT” polymer.Poly(1,4-cyclohexanedimethanol terephthalate) is a polyester thatcontains repeat units from a dicarboxylic acid component and a glycolcomponent. At least about 80 mol percent, more preferably at least about90 mol percent, and especially preferably all of the diol repeat unitsare derived from 1,4-cyclohexanedimethanol and are of formula (I).

At least about 80 mol percent, more preferably at least about 90 molpercent, and especially preferably all of the dicarboxylic acid repeatunits are derived from terephthalic acid and are of formula (II).

In one embodiment, the PCT polymer contains 100 mol percent ofterephthalic acid or diesters. The glycol component, on the other hand,can contain a total of 100 mol percent 1,4-cyclohexanedimethanol.

In various embodiments, however, the dicarboxylic acid component maycontain up to 10 mol percent of other aromatic, aliphatic, or alicyclicdicarboxylic acids such as isophthalic acid, naphthalenedicarboxylicacid, cyclohexanedicarboxylic acid, succinic acid, subacic acid, adipicacid, glutaric acid, azelaic acid, and the like.

The glycol component may also contain up to about 10 mol percent ofother aliphatic or alicyclic glycols, such as diethylene glycol,triethylene glycol, ethylene glycol, propanediol, butanediol,pentanediol, hexanediol, and the like.

The PCT polymer can have an inherent viscosity (I.V.) of from about 0.3to about 1.5 and a melting point of at least 260° C.

In one embodiment, the PCT polymer can comprise a blend of two or moredifferent grades of PCT polymers. For instance, in one embodiment, ablend, such as a 1:1 blend, of high I.V. PCT polymer with a low I.V. PCTpolymer may be used. In an alternative embodiment, a blend, such as a2:1 blend, may be used that includes a PCT polymer wherein thedicarboxylic acid component is 100 mol percent terephthalic acid and aPCT polymer in which the dicarboxylic acid component is 90 mol percentterephthalic acid and 10 mol percent isophthalic acid.

In general, the PCT polymer is present in the composition in an amountof at least about 20% by weight, such as in an amount of at least 30% byweight, such as in an amount of at least 40% by weight, such as in anamount of at least about 50% by weight, such as in an amount of at leastabout 60% by weight. The PCT polymer is generally present in an amountless than about 80% by weight, such as in an amount less than about 70%by weight. In one embodiment, the PCT polymer is present in an amountfrom about 20% by weight to about 60% by weight.

In addition to the PCT polymer, the composition also contains at leastone white pigment in amounts greater than 10% by weight, such as inamounts of at least about 15% by weight. The white pigment is present inthe composition in an amount sufficient to increase the reflectance ofarticles molded from the composition. White pigments that may beincluded in the composition include titanium dioxide, zinc oxide, whitelead, aluminum oxide, barium sulfate, and the like.

In one embodiment, the white pigment comprises titanium dioxide. Thetitanium dioxide may be any sort, such as a rutile titanium dioxide. Thetitanium dioxide particles can have any suitable shape, such asspherical particles or elliptic particles. The titanium dioxide powdercan be comprised of particles having a diameter of from about 10 nm toabout 20,000 nm, such as from about 150 nm to about 500 nm.

In one embodiment, the titanium dioxide particles can be coated. Forexample, the titanium dioxide particles can be first coated with aninorganic coating and then optionally with an organic coating that isapplied over the inorganic coating. Inorganic coatings that may be usedinclude metal oxides. Organic coatings may include carboxylic acids,polyols, alkanolamines, and/or silicon compounds.

Examples of carboxylic acids suitable for use as an organic coatinginclude adipic acid, terephthalic acid, lauric acid, myristic acid,palmitic acid, stearic acid, polyhydroxystearic acid, oleic acid,salicylic acid, malic acid, and maleic acid. As used herein, the term“carboxylic acid” includes the esters and salts of the carboxylic acids.

Examples of silicon compounds suitable for an organic coating include,but are not limited to, silicates, organic silanes, and organicsiloxanes, including organoalkoxysilanes, aminosilanes, epoxysilanes,mercaptosilanes, and polyhydroxysiloxanes. Suitable silanes can have theformula R_(x)Si(R′)_(4-x), wherein R is a nonhydrolyzable aliphatic,cycloaliphatic, or aromatic group having from 1 to about 20 carbonatoms, and R′ is one or more hydrolyzable groups such as an alkoxy,halogen, acetoxy, or hydroxy group, and X is 1, 2, or 3.

Useful suitable silanes suitable for an organic coating include one ormore of hexyltrimethoxysilane, octyltriethoxysilane,nonyltriethoxysilane, decyltriethoxysilane, dodecyltriethoxysilane,tridecyltriethoxysilane, tetradecyltriethoxysilane,pentadecyltriethoxysilane, hexadecyltriethoxysilane,heptadecyltriethoxysilane, octadecyltriethoxysilane, N-(2-aminoethyl)3-aminopropylmethyldimethoxysilane, N-(2-aminoethyl)3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane,3-glycidoxypropyltrimethoxysilane,3-glycidoxypropylmethyldimethoxysilane, 3-mercaptopropyltrimethoxysilaneand combinations of two or more thereof. In other useful silanes, R hasbetween 8 and 18 carbon atoms and R′ is one or more of chloro, methoxy,ethoxy, or hydroxy groups.

One or more white pigments may be present in the composition in anamount of at least about 10% by weight, such as in an amount of at leastabout 15% by weight, such as in an amount of at least 20% by weight,such as in an amount of at least 25% by weight. The white pigments maybe present in the composition generally in an amount less than 60% byweight, such as in an amount less than about 50% by weight.

The polymer composition of the present disclosure can also optionallycontain one or more reinforcing agents, such as fillers and fibers. Suchmaterials can include, for instance, glass fibers, wollastonite,potassium titanate, calcium carbonate, talc, mica, silica, kaolin, andthe like. Such inorganic fillers may be present in the composition in anamount from about 1% to about 40% by weight, such as in an amount fromabout 10% to about 30% by weight.

In accordance with the present disclosure, the composition furthercontains one or more reactive viscosity stabilizers. A reactiveviscosity stabilizer comprises a material capable of not only reactingwith end groups on the PCT polymer, but also capable of stabilizing theviscosity of the PCT polymer in a manner that prevents the viscosityfrom fluctuating during melt processing. The reactive viscositystabilizer can also serve to compatibilize the PCT composition.

In one embodiment, the reactive viscosity stabilizer comprises amaterial that can react with carboxyl or hydroxyl end groups on the PCTpolymer. In this manner, the reactive viscosity stabilizer may act as achain extender.

Reactive viscosity stabilizers that may be used in accordance with thepresent disclosure generally include phenoxy resins and/or non-aromaticepoxy resins. In one embodiment, for instance, the reactive viscositystabilizer comprises a modified phenoxy resin that is capable ofreacting with the PCT polymer. The phenoxy resin, for instance, mayinclude hydroxyl functionality. The phenoxy resin, for instance, mayhave a glass transition temperature of less than about 120° C., such asless than about 110° C., such as less than about 100° C. The phenoxyresin may have a viscosity when tested in cyclohexanone at 25% NV ofless than about 2500 cP, such as less than about 2300 cP.

Non-aromatic epoxy resins that may be used as the reactive viscositystabilizer include 3,4-epoxycyclohexenylmethyl-3′,4′-epoxycyclohexenecarboxylate, 1,4-cyclohexane dimethanoldigrycicdyl ether, hydrogenatedbis-phenol-A type epoxy resin and/or tris(2,3-epoxypropyl)isocyanurate.In general, any suitable alicyclic epoxy resin may be used.

In addition to the above reactive viscosity stabilizers or instead ofthe above reactive viscosity stabilizers, the composition may contain anepoxy-functional copolymer as the reactive viscosity stabilizer.Exemplary copolymers having multiple epoxy pendant groups include thereaction products of one or more ethylenically unsaturated monomers(e.g. styrene, ethylene, and the like) with an epoxy-containingethylenically unsaturated monomer (e.g. glycidyl C1-4 (alkyl)acrylate,ally glycidyl ethacryalte, and glycidyl itoconate). For example, in oneembodiment the epoxy-functional copolymer is a styrene-acrylic copolymer(including an oligomer) containing glycidyl groups incorporated as sidechains.

The one or more reactive viscosity stabilizers are present in thecomposition in an amount sufficient to stabilize the viscosity of thecomposition during melt processing without causing viscosityfluctuations. In general, the reactive viscosity stabilizers are presentin the composition in an amount from about 0.2% to about 8% by weight,such as from about 0.5% to about 5% by weight.

Of particular advantage, reactive viscosity stabilizers are used that donot significantly increase yellowing of the composition over time. Inthis regard, the polymer composition can be formulated so as to besubstantially or completely free of various aromatic epoxy resins. Inone embodiment, for instance, the composition is free of any epoxynovolac resins, such as an epoxy cresol novolac resin.

The polymer composition of the present disclosure can further containone or more impact modifiers. The impact modifiers can be reactive withthe PCT polymer or non-reactive. In one embodiment, for instance, thecomposition contains at least one reactive impact modifier and at leastone non-reactive impact modifier.

Reactive impact modifiers that may be used include ethylene-maleicanhydride copolymers, ethylene-alkyl(meth)acrylate-maleic anhydridecopolymers, ethylene-alkyl(meth)acrylate-glycidyl(meth)acrylatecopolymers, and the like. In one embodiment, for instance, a reactiveimpact modifier is used that comprises a random terpolymer of ethylene,methylacrylate, and glycidyl methacrylate. The terpolymer can have aglycidyl methacrylate content of from about 5% to about 20%, such asfrom about 6% to about 10%. The terpolymer may have a methylacrylatecontent of from about 20% to about 30%, such as about 24%.

Of particular advantage, the present inventors have discovered that thecombination of a reactive impact modifier with a reactive viscositystabilizer may, in some embodiments, further improve the whiteness indexof articles made according to the present disclosure after heat aging.

In general, a reactive impact modifier may be present in the compositionin an amount from about 0.05% to about 10% by weight, such as in anamount from about 0.1% to about 5% by weight.

Non-reactive impact modifiers that may be blended into the polymercomposition of the present disclosure generally include various rubbermaterials, such as acrylic rubbers, ASA rubbers, diene rubbers,organosiloxane rubbers, EPDM rubbers, SBS or SEBS rubbers, ABS rubbers,NBS rubbers, and the like.

In one embodiment, an ethylene acrylic rubber is present such as anethylene acrylic ester copolymer. Particular examples of non-reactiveimpact modifiers include ethylene butylacrylate, ethylene(methyl)acrylate, or 2 ethyl hexyl acrylate copolymers. In oneparticular embodiment, an ethylene (methyl)acrylate copolymer is presentin the composition that contains (methyl)acrylate in an amount of fromabout 20% to about 30% by weight, such as in an amount of about 24% byweight.

In one particular embodiment, the composition of the present disclosureincludes a combination of an ethylene (methyl)acrylate copolymercombined with a terpolymer of ethylene, methylacrylate and glycidylmethacrylate.

When present in the composition, non-reactive impact modifiers can beincluded in amounts of from about 0.05% to about 15% by weight, such asin an amount from about 0.1% to about 8% by weight.

The polymer composition of the present disclosure may contain variousother thermoplastic polymers in addition to the PCT polymer. The otherthermoplastic polymers can be present in an amount from about 1% toabout 15% by weight. Other thermoplastic polymers that may be includedinclude other polyester polymers, a liquid crystal polymer, or mixturesthereof. Other thermoplastic polyester polymers that may be included inthe composition include poly(ethylene terephthalate), poly(propyleneterephthalate), poly(butylene terephthalate), acid-modified PCTcopolyesters, poly(ethylene naphthalate), poly(butylene naphthalate),aliphatic polyesters such as polyester glutarate, and the like. Theinclusion of small amounts of other polyester polymers or a liquidcrystal polymer may, in some embodiments, improve the processability ofthe composition. In one embodiment, for instance, the composition maycontain an aromatic liquid crystal polyester polymer in an amount offrom about 2% to about 15% by weight.

Another additive that may be present in the polymer composition is apolytetrafluoroethylene polymer. Inclusion of a polytetrafluoroethylenepolymer may enhance the reflectance and the whiteness index of articlesmade from the polymer composition. The polytetrafluoroethylene polymermay be added to the composition in the form of a fine powder having anaverage particle size of less than about 50 microns, such as less thanabout 10 microns. In one embodiment, for instance, thepolytetrafluoroethylene powder may have an average particle size of fromabout 1 micron to about 8 microns. The polytetrafluoroethylene polymermay be present in the composition in an amount from about 0.05% to about10% by weight, such as from about 0.1% to about 6% by weight.

In one embodiment, the polymer composition can also include a lubricant.The lubricant may comprise, for instance, a polyethylene wax, an amidewax, a montanic ester wax, a polyol ester, or the like. A lubricant, incertain embodiments, for instance, may comprise a polyethyleneglycol-dilaurate and/or a neopentyl glycol dibenzoate. In one particularembodiment, the lubricant may comprise an oxidized polyethylene wax. Thepolyethylene wax may have a density of from about 0.94 g/cm³ to about0.96 g/cm³. When present, the lubricant may be included in thecomposition in an amount from about 0.05% to about 6% by weight, such asfrom about 0.1% to about 4% by weight.

In addition to the above, the polymer composition may contain variousother additives and ingredients. For instance, the composition maycontain various thermal and oxidative stabilizers, ultraviolet lightstabilizers, brighteners, and the like. Examples of various otheringredients include sterically hindered phenolic antioxidants, phosphiteantioxidants and the like.

In one embodiment, the polymer composition may contain a stericallyhindered amine light stabilizer. When present in the composition,hindered amine light stabilizers have been found to provide variousadvantages and benefits. For instance, sterically hindered amine lightstabilizers have been found to further improve the reflectanceproperties of the material, especially after long term aging. Lightstabilizers may be present in the composition in an amount from about0.05% to about 3% by weight, such as in an amount from about 0.05% toabout 1% by weight. In one particular embodiment, a hindered amine lightstabilizer may be used in conjunction with a hindered phenolicantioxidant and a phosphite stabilizer.

In order to produce articles in accordance with the present disclosure,the polymer composition, in one embodiment, can comprise a melt-mixedblend, wherein all of the polymeric components are well-dispersed withineach other and all of the non-polymeric ingredients are well-dispersedin and bound by the polymer matrix, such that the blend forms a unifiedwhole.

Any melt-mixing method may be used to combine the polymeric componentsand non-polymeric ingredients. For example, in one embodiment, thepolymeric components and the non-polymeric components may be added to amelt mixer, such as for example a single or twin-screw extruder, ablender, a kneader, or a Banbury mixer, either all at once through asingle step addition, or in a stepwise fashion and then melt-mixed.

The blended composition can then be molded into any desired shapethrough any suitable molding process. For instance, in one embodiment,articles are formed through injection molding. During injection molding,the temperature of the composition may be from about 280° C. to about350° C. The temperature of the molds, on the other hand, may be in arange of from about 80° C. to about 150° C.

In one embodiment, the melt viscosity of the polymer composition whenmeasured by a capillary rheometer at 305° C. under 1000 sec⁻¹ shear rateis from about 60 Pa·s to about 250 Pa·s, such as from about 70 Pa·s toabout 200 Pa·s. In one embodiment, for instance, the melt viscosity maybe from about 80 Pa·s to about 160 Pa·s.

As described above, the PCT polymer composition of the presentdisclosure is particularly well suited for producing reflectors for LEDassemblies. The reflectance properties of the polymer are particularlywell suited for use with white LEDs. The LED reflector may be in theform of a single piece or may be formed from two or more subparts. Inone embodiment, the polymer composition is injection molded over thelead frame as shown in FIGS. 1 and 2. In this manner, the lead frame andthe reflector become integrated together. The semiconductorlight-emitting diode chip can then be mounted within the cavity of thereflector and connected to the lead frame. The LED can be bonded to thelead frame using the bonding wires. The entire assembly can be encasedor the cavity defined by the reflector can be filled with a corematerial such as a solid epoxy that can form a lens for focusing thelight in a single direction.

LED assemblies made in accordance with the present disclosure can beused in numerous and different applications. For instance, the LEDassemblies can be used in traffic signal lights, LCD displays,backlights, cellular telephones, automotive display lights, automotiveheadlamps, flashlights, interior lighting, streetlights, and in exteriorlighting applications.

The present disclosure may be better understood with reference to thefollowing examples.

EXAMPLES

The following examples are presented below by way of illustration andnot by way of limitation.

Table 1 below lists various compositions that were prepared by meltcompounding the components shown in the table using a 32 mmtwin-extruder operating at 300° C., using a screw speed of about 350 rpmand a melt temperature of from about 320° C. to about 330° C. Uponexiting the extruder, the compositions were cooled and pelletized.

The compositions were molded into ISO tensile bars according to ISOMethod 527-1/2 using a mold temperature of about 120° C. Tensileproperties of the samples were determined using the test method above.Charpy impact strengths and Notched Charpy impact strengths weredetermined following ISO Test 179.

Initial reflectance at 460 nm was determined for each composition usingASTM Test Method E1331 using a CIE D65 daylight illuminant at 10° by aspectracolormeter DataColor 600. Measurements were done on the tensilebars. A higher reflectance number indicates less absorption or loss oflight.

The initial whiteness index and the whiteness index after aging weredetermined using the same reflectance scan based on WI E313. Higherwhiteness index numbers indicate better whiteness.

The following results were obtained:

Example Example Example Example Example Example Example Example ExampleComposition (wt %) 1 2 3 4 5 6 7 8 9 PCT polymer 54.5 47.4 51.9 46.949.9 47.9 47.8 40.8 43 Chopped Glass Fiber 20 16 16 16 16 16 18 18 16Titanium Dioxide 25 25 20 25 20 20 20 20 20 Oxidized polyethylene wax0.5 0.5 0.5 0.5 0.5 0.4 0.4 0.5 Phenolic antioxidant 0.2 0.25 0.25 0.250.25 0.25 0.03 0.03 0.3 Phosphite stabilizer 0.3 0.3 0.3 0.3 0.3 0.30.03 0.03 Hindered amine light stabilizer 0.44 0.44 0.15 Epoxy cresolnovolac resin 0.5 Benzoxazole,2,2′-(1,2- 0.05 0.05 0.05 0.05 0.05 0.10.1 0.05 ethenediyldi-4,1-phenylene)bis Ethylene, methyl acrylate, 1 10.5 0.5 0.5 glycidyl methacrylate terpolymer Phenoxy resin 2 1 2 2 1 1 1Polytetrafluoroethylene 2 4 3 powder Ethylene-methylacrylate 3 3 3 3 3 33 1.5 copolymer Talc 2 2 2 2 2 2 2 2 N,N′ ethylene bisstearamide 0.7 0.71 Polybutylene terephthalate 4 4 4 4 4 3 3 8 Liquid Crystal Polymer 10 6Total (wt %) 100 100 100 100 100 100 100 100 100 Properties TensileStrength (Mpa) 61 73 84 76 83 86 69 86 74 Elongation at Break (%) 0.91.9 1.8 1.9 1.8 1.7 2.0 1.7 1.6 Notched Charpy Impact (KJ/m²) 2.5 4.83.9 4.1 3.9 3.9 3.6 3.6 3.2 Unnotched Charpy Impact 19 38 32 36 34 33 3525 25 (KJ/m²) R % at 460 nm 90 91 96 95 96 97 97 93 92 Initial WhitenessIndex 81 84 97 93 96 96 99 90 87 Whiteness Index after aging at 37 26 7278 70 75 62 65 66 200° C. for 4 hr WI Retention after aging (%) 45.731.0 74.2 83.9 72.9 78.1 62.6 72.2 75.9

As shown in the table above, the polymer composition of Example 1 didnot contain a reactive viscosity stabilizer. Example No. 2, on the otherhand, contained an epoxy cresol novolac resin. In Example 2, thewhiteness index after aging was severely reduced.

Compositions made according to the present disclosure, on the otherhand, displayed excellent initial reflectance, excellent initialwhiteness index and displayed excellent whiteness index properties afteraging.

These and other modifications and variations to the present inventionmay be practiced by those of ordinary skill in the art, withoutdeparting from the spirit and scope of the present invention, which ismore particularly set forth in the appended claims. In addition, itshould be understood that aspects of the various embodiments may beinterchanged both in whole or in part. Furthermore, those of ordinaryskill in the art will appreciate that the foregoing description is byway of example only, and is not intended to limit the invention sofurther described in such appended claims.

1. A molded reflector surrounding a light-emitting source, the reflectorbeing molded from a polymeric material, the polymeric material beingcomprised of a poly(1,4-cyclohexanedimethanol terephthalate), a whitepigment, and at least one reactive viscosity stabilizer, the reactiveviscosity stabilizer comprising a phenoxy resin or a non-aromatic epoxyresin, the polymeric material being free of epoxy novolac resins, thepolymeric material having an initial reflectance at 460 nm of greaterthan about 90%, having an initial whiteness index of greater than about86, and having a whiteness index after aging at 200° C. for four hoursof greater than about
 50. 2. A molded reflector as defined in claim 1,wherein the polymeric material further contains an inorganic filler. 3.A molded reflector as defined in claim 2, wherein the polymeric materialcontains from about 20% to about 60% of thepoly(1,4-cyclohexanedimethanol terephthalate), from about 1 to about 40%by weight of the inorganic filler, from about 15% to about 50% by weightof the white pigment, and from about 0.2% to about 8% by weight of thereactive viscosity stabilizer.
 4. A molded reflector as defined in claim1, wherein the reactive viscosity stabilizer modifier contained withinthe polymeric material comprises a phenoxy resin.
 5. A molded reflectoras defined in claim 3, wherein the reactive viscosity stabilizermodifier contained within the polymeric material comprises a phenoxyresin.
 6. A molded reflector as defined in claim 1, wherein the reactiveviscosity stabilizer contained within the polymeric material comprisesan alicyclic epoxy resin.
 7. A molded reflector as defined in claim 3,wherein the reactive viscosity stabilizer contained within the polymericmaterial comprises an alicyclic epoxy resin.
 8. A molded reflector asdefined in claim 1, wherein the polymeric material has a notched impactstrength of from about 3 KJ/m² to about 5 KJ/m².
 9. A molded reflectoras defined in claim 1, wherein the reactive viscosity stabilizers arepresent in the polymeric material in an amount from about 0.2% to about5% by weight.
 10. A molded reflector as defined in claim 1, wherein thepolymeric material further comprises at least one more thermoplasticpolymer, the thermoplastic polymer comprising a polybutyleneterephthalate, a liquid crystal polymer, or mixtures thereof, the atleast one thermoplastic polymer being present in the polymeric materialin an amount from about 1% to about 15% by weight.
 11. A moldedreflector as defined in claim 1, wherein the polymeric material furthercontains a polytetrafluoroethylene polymer, the polytetrafluoroethylenepolymer being present in the polymeric material in an amount from about1% to about 10% by weight.
 12. A molded reflector as defined in claim 1,wherein the polymeric material further contains a reactive impactmodifier and a non-reactive impact modifier.
 13. A molded reflector asdefined in claim 1, wherein the polymeric material contains a terpolymerof ethylene, methyl acrylate and glycidyl(meth) acrylate.
 14. A moldedreflector as defined in claim 1, wherein the polymeric material furthercontains an ethylene-(methyl)acrylate copolymer.
 15. A molded reflectoras defined in claim 6, wherein the polymeric material has a whitenessindex after aging at 200° C. for four hours of greater than about 60.16. A molded reflector as defined in claim 1, wherein the polymericmaterial exhibits a spiral flow of greater than 5 inches.
 17. A moldedreflector as defined in claim 1, wherein the reactive viscositystabilizer comprises a copolymer having multiple epoxy pendant groups.18. A composition for producing molded parts having improved reflectiveproperties comprising: from about 20% to about 60% by weight of apoly(1,4-cyclohexanedimethanol terephthalate); from about 1% to about40% by weight of an inorganic filler; from about 15% to about 50% byweight of a white pigment; from about 0.5% to about 8% by weight of oneor more reactive viscosity stabilizers, wherein at least one of thereactive viscosity stabilizers comprises a phenoxy resin or anon-aromatic epoxy resin; and wherein the composition is free of epoxynovolac resins and wherein the composition has a spiral flow of greaterthan 5 inches.
 19. A composition as defined in claim 18, wherein thecomposition further comprises a polybutylene terephthalate, a liquidcrystal polymer, or mixtures thereof.
 20. A composition as defined inclaim 18, wherein the composition further contains apolytetrafluoroethylene polymer in an amount from about 1% to about 10%by weight.
 21. A composition as defined in claim 18, wherein thecomposition further contains an ethylene-(methyl)acrylate copolymer. 22.A composition as defined in claim 18, wherein the composition furthercontains a terpolymer of ethylene, methyl acrylate and glycidyl(meth)acrylate.
 23. A composition as defined in claim 21, wherein thecomposition further contains a terpolymer of ethylene, methyl acrylateand glycidyl(meth) acrylate.
 24. A composition as defined in claim 18,wherein the reactive viscosity stabilizer comprises an epoxy-functionalcopolymer comprising a styrene-acrylic copolymer containing glycidylgroups incorporated as side chains.